The meridian directly opposite the Prime one ... roughly down the middle of the Pacific Ocean ... is the meridian of 180° longitude, both East and West.
Technically, it should also be the International Date Line, but it's not. The International Date Line has been defined with several kinked segments in it, in order to avoid having it cut through the few islands that lie along 180°.
If it were allowed to do that, there would be nations that would have two different calendar dates on the same island. Kids who live on one side and go to school on the other side would constantly be either cutting school or showing up on Saturday, and people would be missing their dental appointments on the other side. Much easier just to put a kink in the International Date Line so that whole country is on the same calendar date.
